Finally had the chance to drop by ACE and buy a contact cleaner. Sprayed generously on all switches, knobs, and speaker connectors, keeping my fingers crossed while waiting for it to completely dry. It worked, the speakers are now balanced! Thanks Tempter, qguy, and jay john!!
On a side note, from time to time I would get a scratchy sound before and the previous owner told me to just turn the selector knob a few times and it fixes the problem. I bought a contact cleaner and sprayed on the knob, but it did not fix the problem. Today I found out that the selector switch I was spraying is just a mechanical switch with a cord that moves the actual switch located near the back. I was cleaning the wrong switch before! I will observe if the previous problem has been fixed.
Fentom, have you tried cleaning?